Such phase shifters may be used, in particular, in array type antennas which include components for adjusting the beam emitted or received by said antennas, the components themselves comprising sets of unit modules spaced apart at an array pitch for adjusting the phases and the amplitudes of the microwave signals they receive.
In space applications, and more particularly for space equipment using miniaturized versions of array type antennas that are of the so-called "active antenna" type, it is necessary to obtain as great a degree of miniaturization as possible in said unit modules, and in particular in their unit circuits for phase adjustment, while retaining adjustment accuracy that is as fine as possible over as large as possible a range of phase shifts.
Other criteria required by this type of application consist in reducing the power consumption of the unit circuits as much as possible and in matching their impedances as closely as possible so as to make it possible to interconnect them directly in cascade with other circuits without using intermediate isolating circuits.
Existing phase shifters for microwave signals can be classified into two categories:
analog phase shifters capable of providing continuous variation in phase shift by acting on a control voltage; and
digital phase shifters capable of providing stepwise variation of phase shift through steps of fixed predetermined value.
Digital phase shifters can readily be made using monolithic integrated technology, and thus satisfy the above-mentioned miniaturization requirement, but as mentioned above they do not provide sufficiently fine adjustment of phase shift.
Analog phase shifters can themselves be classified into a plurality of categories:
"vector modulation" phase shifters based on the principle of splitting the input signal into four equal fourths at mutual 90.degree. phase shifts, thereby creating four same-amplitude orthogonal vectors. Thereafter, each of these vectors is attenuated by a variable attenuator and then the four signals obtained in this way are recombined and the resulting output signal is at a given phase shift relative to the input signal, which shift thus depends on the values of the four attenuations applied to the signal. The drawbacks of this solution lie in the complexity of the circuit (requiring three dividers, four attenuators, and three combiners), in the lack of accuracy in the final phase shift, and above all in the DC power consumption of the circuit being far from being negligible which makes it quite unsuitable for space applications;
"reflection" phase shifters based on the principle of splitting the input signal over the direct and coupled paths of a 3 dB hybrid coupler (of the Lange's coupler type). Identical L-C cells (made up of inductors and variable capacitance capacitors constituted by varactor diodes) are placed on said paths and they reflect the signals which recombine at the outlet with a certain phase shift. The main drawback of this topology lies in the large area occupied by the circuit which makes it rather difficult to integrate into monolithic technology; and
phase shifters based on varying the angle of the transmission coefficient of a transistor having negative feedback provided by means of a variable reactance.
A phase shifter of that type is described in the document: 18th European Microwave Conference--Stockholm 1988 "A novel microwave transmission phase shifter" by J. P. Coupez and R. A. Perichon. The variable reactance is formed by connecting in series an inductance and a variable capacitance capacitor constituted by a varactor diode to make a monolithic integrated circuit implementation possible. However, that phase shifter does not include associated inlet and outlet matching circuits which means that its performance in practice is quite mediocre.
FIG. 1 is a Smith chart showing various values for the inlet reflection coefficient S11 and the outlet reflection coefficient S22 of such a phase shifter without inlet and outlet matching circuits. These various values are obtained over a set of control voltage values for the varactor diodes constituting the variable capacitance capacitors (five values in this case), and at each of these values, frequency is also varied over a certain range giving rise to a set of curved arcs in FIG. 1 which show the way in which said reflection coefficients vary.
An object of the present invention is to provide such inlet and outlet matching circuits that are effective over the entire range of reactance variation in the negative feedback of the transistor for a given range of operating frequencies.